The invention applies by way of example, but not exclusively, to the nuclear fuel used in light water reactors, such as pressurized water reactors.
The nuclear fuel used in such reactors is conditioned in the form of pellets.
These pellets are placed in cladding to form nuclear fuel rods that are grouped together within assemblies. Such assemblies are for loading in the cores of nuclear reactors.
In order to fabricate nuclear fuel pellets, nominal values and individual tolerances are set for fabrication parameters. In fabricated pellets, the real values of these fabrication parameters necessarily vary from one pellet to another relative to the nominal value, and for each pellet these values are required to comply with selected fabrication tolerances.
The design of fuel pellets is subject to imperative safety rules of the kind that characterize the entire nuclear industry.
In this context, it is appropriate to make effective use of fissile material, while requiring the manufacturer to comply with specifications that are pertinent, and while also presenting the operator with operating conditions that are flexible.
Satisfying these objectives requires detailed knowledge about the behavior of the nuclear fuel and requires inspection of the variations that necessarily occur in fabrication relative to nominal values for fabrication parameters.
The parameters characterizing nuclear fuel pellets are too numerous to enable them to be taken into account directly when performing safety studies.
Neutron calculations implemented in such safety studies are therefore performed using nominal values, and the variations relative to these values are taken into account by means of multiplier coefficients that are referred to as technological uncertainty factors and that are applied to the results of the neutron calculations.
Use is made in particular of the technological uncertainty factor for the linear power density at the hot point FQE as defined below.
For a given nuclear reactor, core safety requires the linear power density of the hottest rod (power peak) to remain at all times below the limit set for reactor safety. It is therefore appropriate to ensure that the linear power density of the hottest rod, as calculated using fabrication parameters at their nominal values and then increased by the factor FQE remains below said limit.
In the past, the factor FQE has been calculated fabrication parameter by fabrication parameter, taking into consideration the permitted value that maximizes linear power density, and even though certain compensation phenomena exist. Furthermore, the uncertainty in that calculation is such that it does not enable different variations to be compared.
To evaluate uncertainties better, and thus make it possible to give flexibility to the nuclear fuel manufacturer or to the reactor operator, or indeed to relax constraints so as to increase the performance of the nuclear fuel, proposals have been made to take account of the random nature of fabrication variations by relying on the generalized and conventional perturbation theory (GCPT).
That method of determining the factor FQE is described, for example, in the Ph.D. thesis presented to Université Claude Bernard—Lyon I, by Guy Willermoz, defended on Sep. 28, 1994 before the Examination Commission and entitled “Etude stochastique de l'impact neutronique des hétérogénéités en fabrication du combustible nucléaire” [A stochastic study of the neutron impact of fabrication non-uniformities in nuclear fuel].
That method makes it possible to quantify the influence of each fabrication parameter on the core state of a nuclear reactor. It was developed by analyzing variations in fabrication parameters in a fuel based on a mixture oxide fuel (MOX).
For this purpose, a probability relationship is associated with different variations in the fabrication parameters of pellets. By using statistical studies to estimate the combined relationship of the fabrication parameters, the influence thereof on the point power relationship can be studied by using Boltzmann's equation.
The stochastic Taylor development was used, with sensitivity coefficients in a multi-parameter context being calculated using the generalized and conventional perturbation theory (GCPT).
Thus, in that prior art method, account is taken of individual variations in the fabrication parameters, i.e. variations from pellet to pellet by multiplying them by sensitivity coefficients that are said to be “microscopic” since they apply to individual variations only. The individual variations taken into account may be the tolerances that each pellet is required to comply with.
That method of determining the technological uncertainty factor FQE has been found to be satisfactory.
Nevertheless, it would be desirable to further improve the determination of technological uncertainty factors such as FQE and thus to increase flexibility for the nuclear fuel manufacturer or for the nuclear reactor operator.